Abstract
Photocatalytic reactions occur at the crystal–solution interface, and hence specific crystal facet expression and surface defects can play an important role. Here we investigate the structure-related photoreduction at zinc oxide (ZnO) microparticles via integrated light and electron microscopy in combination with silver metal photodeposition. This enables a direct visualization of the photoreduction activity at specific crystallographic features. It is found that silver nanoparticle photodeposition on dumbbell-shaped crystals mainly takes place at the edges of O-terminated (0001̅) polar facets. In contrast, on ZnO microrods photodeposition is more homogeneously distributed with an increased activity at {101̅1̅} facets. Additional time-resolved measurements reveal a direct spatial link between the enhanced photoactivity and increased charge carrier lifetimes. These findings contradict previous observations based on indirect, bulk-scale experiments, assigning the highest photocatalytic activity to polar facets. The presented research demonstrates the need for advanced microscopy techniques to directly probe the location of photocatalytic activity.
Highlights
The knowledge of the facet-dependent photocatalytic performance of zinc oxide (ZnO) is mostly based on indirect evidence obtained from ensemble-averaged experiments
The UV photodeposition of silver on ZnO crystals is often used to improve the photocatalytic performance as the silver acts as an efficient trap for the photogenerated electrons, reducing the probability of electron−hole pair recombination.[20−23] so far, the facet-dependent photodeposition of silver nanoparticles has not been studied in great detail.[24−26] In this work, the photoinduced silver deposition is explored at the single-particle level for two commonly encountered ZnO structures with different crystallographic facets using an integrated light and electron microscope.[27,28]
An optical microscope was integrated into a FEI Quanta 250 FEG environmental scanning electron microscope (SEM) using the SECOM platform of Delmic (Scheme S1).[29,30]
Summary
Observation of ZnO crystals by SEM, before and after UVinduced silver cocatalyst deposition.[28]. Similar to the ZnO dumbbells, time-resolved PL studies on individual ZnO microrods (Figure 3b) demonstrate increased free chargecarrier lifetimes near the O-terminated polar facets, indicating an enhanced charge-carrier diffusion toward these crystal facets In their pioneering work, Pacholski et al.[24] demonstrated that the photodeposition of silver on very small ZnO nanocrystals. To minimize the effect of preferential electrostatic attraction of silver cations, the described experiments were performed at pH 10 as the bulk isoelectric point of ZnO was determined to be at pH 8.7 to 10.3.35 Protonation of the O-terminated polar face was reported to induce a downward band bending, decreasing the recombination probability of the photogenerated charges and inducing a directional electron diffusion toward this polar facet.[35,40] This model has been confirmed by our experimental results, as they reveal longer charge carrier lifetimes at the outlines of O-terminated (0001̅) facets and structural imperfections, as previously discussed (Figure 3).
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